ON THE ORIGIN OF SPECIES. 489 



species do not show any or, at least, no constant differences, the above- 

 mentioned couples of species, and compound species excepted. Spruces 

 form a compound species, consisting of numerous types, but the com- 

 mon fir which without doubt is older than our era has remained the 

 same everywhere. It is ever thus; the species do not undergo any 

 gradual change, but each species is constant and remains so until 

 others take its place. It never or but rarely occurs that new species 

 make their appearance in fully investigated countries, unless indeed 

 they happen to have been introduced from elsewhere. Yet it is prob- 

 able that new species are formed quite frequently, but that, being too 

 weak, they succumb before one becomes aware of their existence. 



The numerous small species which are united under the name 

 Draba verna are constant to seed, they do not change, besides they are 

 distributed throughout Europe. It is therefore considered probable 

 that there was a time during which they were formed, probably in a 

 comparatively small region in the central part of Europe [at the pres- 

 ent day they are most frequent along the Khine and the Loire], and 

 that in this locality flourished one or more species from which the 

 present forms originated. At the end of this mutation-period the spe- 

 cies would again have become constant. In this manner mutable and 

 immutable periods in the development of species would have alternated 

 more or less regularly. 



There is a great tendency to consider a rapid increase in number 

 as one of the reasons which cause a species to become temporarily 

 mutable. Many species multiply exceedingly rapidly when they are 

 transported to a new region where the conditions are favorable. Many 

 European plants did this in America, likewise many American plants 

 in Europe, as is only too well known through the waterpest, Elodea 

 canadensis. As a matter of fact, we did not see them 'mutate,' but 

 this may have been due to insufficient observation. It would be of 

 great importance to pay close attention to this point when draining 

 lakes, clearing waste lands, after forest fires and in similar cases. 



Whether the mutations, during the mutable period, have been one- 

 sided or many-sided, is a most important question and one frequently 

 discussed by the adherents of the mutation theory. The case of Draba 

 verna, just mentioned, certainly speaks in favor of many-sided varia- 

 bility; the 200 'subspecies' known, vary in all organs and in all pos- 

 sible ways. Numerous other instances might be quoted. But opposed 

 to them are the results obtained by paleontology. The progress in 

 zoological times, more particularly in the animal kingdom, has always 

 followed definite lines; by a straight line nature tried to reach her 

 goal, not by zigzag lines, feeling her way. The main line has of 

 course numerous small side branches, but branches which do not lead 

 to still living types are rare. Scott and others deduce from this that 

 mutability is one-sided, only progressing in the desired direction. Yet 



